Disentangling seasonal and annual precipitation signals in the tropics over the Holocene: insights from δD, alkanes and GDGTs

Tropical Asia is a critical component of the global climate system as it provides large amounts of moisture and heat to the extratropics and drives monsoons on both hemispheres. Paleoclimate information remains relatively scarce from the region despite its global significance, calling for additional records from the region. We generated a multiproxy peat record from Sumatra, with a focus on deconvolving seasonal and annual precipitation signals, as well as long term temperature variability. We do this by analyzing the n-alkane distributions reflecting vegetation variability, and the hydrogen isotopic composition (δD_alkanes) from alkanes sourced from algae and terrestrial plants. Algae grows during the wet season, while terrestrial plants grow over the whole year, and they therefore reflect the water isotopic signal from different parts of the year. We further analyze the GDGT composition in the peat core to derive information about temperature and hydrological changes. Finally, levoglucosan was measured to reconstruct past wildfire events.

We find that the climate on Sumatra was much more seasonal in the Mid-Holocene than in the Late Holocene, based on the difference between δD_terrestrial and δD_algae. In particular, the period between 4-6 ka BP was extremely seasonal, with alternating floods, droughts and fires. This extreme seasonality is coeval with an Asian Summer Monsoon collapse, Australian Summer Monsoon invigoration and the collapse of Green Sahara, suggesting large scale tropical atmospheric reorganization in that period.

Our multiproxy annual precipitation reconstruction indicates the wettest overall conditions between 3.3-4.5 ka BP. which is approximately 1500-2000 years later than indicated by a nearby speleothem δ¹⁸O record, which instead is more similar to δD_algae. We therefore hypothesize that speleothem reconstructions in the region record a wet season isotopic signal, similar to the algae, since cave groundwater recharge occur mainly after heavy precipitation. The Late Holocene is marked by rapid drying around 2.8 ka BP, under a much less seasonal climate, which is coeval with the strengthening of ENSO variability in the Pacific Ocean.

In summary, our multiproxy peat record from Sumatra resolves the seasonal versus annual components of past rainfall variability, revealing heightened seasonality during the Mid-Holocene, significant shifts in precipitation pattern, and a notable Late Holocene drying trend. Our findings highlight the importance of considering the δD signal from the full range of alkanes and considering seasonal variability in paleoclimatological reconstructions.